Tumor oxygenation is an important variable in treatment and tumors exhibit a wide range of oxygen tensions. We hypothesized that non-invasive MR imaging of the ADC may be useful for assessing tumor oxygenation and correlated ADC with tumor pO2 in mouse models. The ADC relates in part to intra/extracellular volume. In cases of chronic hypoxia, but before frank necrosis has occured, the cells may swell due to impaired ion regulation. This would result in a decreased ADC. The relationship was determined in RIF-1 tumors and, as hypothesized, the ADC was found to increase with increasing pO2.